Coupling device

ABSTRACT

A coupling device has a pulley that receives rotational torque from an engine. The torque is transmitted to a shaft through a hub and a one-way clutch. A torque limiter is screwed on the shaft to fasten the inner ring of the one-way clutch onto the shaft. The torque limiter has a breakable portion. The torque is directly transmitted from the inner ring to the shaft when the torque is normal. If the torque exceeds the tightening torque of the torque limiter, the inner ring begins to slide and rotate, and drives the torque limiter in a tightening direction. As the torque limiter is tightened additionally, the tensile stress on the breakable portion is increased. Then, the breakable portion breaks and is separated in axial direction so that the coupling device disconnects torque transmission.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application is based on Japanese Patent Applications No.2002-113514 filed on Apr. 16, 2002 and No. 2003-57078 filed on Mar. 4,2003 the contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to a coupling device fortransmitting torque.

[0004] 2. Description of Related Art

[0005] JP-A-8-319945 discloses a compressor with coupling device. Thecoupling device transmits torque. The coupling device is formed as apulley for receiving rotational torque from a belt. The coupling deviceis also capable of functioning as a torque limiter to disconnect torquetransmission when the torque exceeds a predetermined threshold torque.The coupling device is formed with a weak portion that breaks todisconnect torque transmission when the torque exceeds the threshold.The weak portion is provided on a disk portion of the pulley by formingan annular groove and through holes on the groove in regular intervals.The through hole weakens and adjusts the strength of the disk. Thisconfiguration is advantageous for providing a coupling device withsimple structure and at less cost. However, contrary, it is difficult todesign and manufacture products having precision break torque.

[0006] For instance, a breakable portion expected to be broken has to bedesigned to have a certain strength that is capable to breaking at acertain break torque, threshold torque. However, it is difficult todetermine the strength of the breakable portion. For example, thefatigue failure may breaks the breakable portion even if it is appliedwith a lower toque than the expected break torque.

[0007] Therefore, the maximum torque applied to the breakable portion,that is a permissible torque T2, should be set below a torque indicatedby T1/S, where T1 is the break torque and S is a safety margin. In thiscase, the coupling device will be useless since the requiredtransmissible torque T3 exceeds the permissible torque T2 when a ratioT1/T3 is lower than the safety ratio S. The required transmissibletorque T3 may be referred to as a torque to be transmitted via thecoupling device under an actual application. In general, it is necessaryto set a greater value in the safety margin S as it becomes difficult tocarry out a theoretical stress analysis of the object since condition ofstress applied to the abject requires complicated analysis. In contrast,if the stress applied to the breakable portion can be estimatedprecisely, it is possible to set a smaller value in the safety margin S,and to increase the permissible torque T2.

[0008] The structure disclosed in JP-A-8-319945 is expected to be brokenprimarily by a shearing force, that is the tangential stress. Since theshearing force tends to intense to a surface, it is difficult toestimate a distribution of the stress precisely. Therefore, a trial anderror method is required to determine appropriate size and material ofthe breakable portion.

SUMMARY OF THE INVENTION

[0009] It is an object of the present invention to provide an improvedstructure of a coupling device.

[0010] It is another object of the present invention to provide acoupling device with a breakable portion that is mainly broken by thetensile stress generated by an excessive torque.

[0011] It is another object of the present invention to provide acoupling device with a breakable portion that is almost free from themetal fatigue failure caused by the fluctuation of the transmittedtorque.

[0012] It is another object of the present invention to provide acoupling device that is capable of disconnecting at a predeterminedtorque without loosening threads and the metal fatigue failure.

[0013] According to a first aspect of the present invention, a couplingdevice is disposed between a drive member and a driven member. Thecoupling device comprises a torque limiter that is rotatable with thedrive member. The torque limiter has a thread formed in a direction togenerate a thrust force when the thread is rotated in the same directionas a direction of a torque transmitted from the drive member to thedriven member, and a breakable portion having a tensile strength thatbreaks at a predetermined level of the thrust force.

[0014] In this structure, the breakable portion is capable of breakingby the tensile force generated by the thread. Therefore, it is possibleto set the break torque precisely.

[0015] The coupling device may additionally comprise a one-way clutchthat prevents the thread from loosening by a torque in a counterdirection. It is possible to prevent a loosening of the thread eventhere is a torque in a direction counter to the direction from the drivemember to the driven member.

[0016] The breakable portion may have the tensile strength that is lessthan the other portions of the torque limiter.

[0017] The torque limiter may be fixed on the drive member. In thiscase, the thread may be adapted to fasten the drive member on the drivenmember in a slidable manner. According to this structure, the drivemember and the driven member provide a frictional contact. Thefrictional contact transmits the transmitted torque from the drivemember to the driven member until the transmitted torque prevails thefrictional torque. If the transmitted torque prevails the frictionaltorque, the drive member is enabled to slide on and rotate relative tothe driven member. Therefore, the torque limiter is almost free from thetransmitted torque and the fluctuation of the transmitted torque duringthe frictional contact transmits the transmitted torque. Therefore, itis possible to prevent the breakable portion from the metal fatiguefailure caused by the fluctuation of the transmitted torque.

[0018] The thread may be formed to make the drive member comes incontact with the driven member in a manner that the drive memberdirectly transmits torque to the driven member when the transmittedtorque is less than a predetermined level, and that the drive member isenabled to rotate with respect to the driven member when the transmittedtorque exceeds the predetermined level.

[0019] The torque limiter may be formed in a cylindrical shape. In thiscase, the torque limiter has a first portion to be fixed on the drivemember, and a second portion to be threaded on the driven member. Thebreakable portion is disposed between the first and second portions. Itis possible to provide a compact structure of the torque limiter.

[0020] The torque limiter may be threaded on the drive member until thetorque limiter and the drive member comes in an axial contact. It ispossible to fix the torque limiter on the drive member firmly.

[0021] The coupling device may further comprise a member that urges thetorque limiter opposite to the tightening direction of the thread. It ispossible to reduce noise and friction after the torque limiter isbroken.

[0022] The drive member may include a first one-way clutch thatselectively transmits a torque from a primary drive source to the drivenmember, and a second one-way clutch that selectively transmits a torquefrom a secondary drive source to the driven member. In this structure,the torque limiter is disposed to be rotated by the torque from theprimary or secondary drive source.

[0023] According to the present invention the torque limiter has acontact surface disposed to contact the driven member, and the thread isprovided to generate a trust force in a contacting direction forpress-contacting the driven member and the contact surface when thethread is rotated in the same direction as a direction of torquetransmitted from the drive member to the driven member. Further, thebreakable portion is disposed between the contact surface and a part ofthe thread, and the breakable portion breaks when the torque is largerthan a predetermined value. Accordingly, the breakable portionaccurately breaks at the torque larger than the predetermined torque.

[0024] Preferably, the contact surface of the torque limiter contactsone end of the driven member, at least one of the contact surface andthe one end of the driven member has a film on its surface, and the filmrestricts a variation in coefficient of friction when the one end of thedriven member contacts the contact surface. Alternatively, a spacer hasa surface that restrict a variation in coefficient of friction, and thespacer is disposed between the contact surface of the torque limiter andthe driven member. Therefore, the breaking is not affected by friction,and the breakable portion accurately breaks at a predetermined torquelevel.

BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Features and advantages of embodiments will be appreciated, aswell as methods of operation and the function of the related parts, froma study of the following detailed description, the appended claims, andthe drawings, all of which form a part of this application. In thedrawings:

[0026]FIG. 1 is a cross sectional view of a coupling device according toa first embodiment of the present invention;

[0027]FIG. 2 is an enlarged cross sectional view of the coupling deviceaccording to the first embodiment of the present invention;

[0028]FIG. 3 is a cross sectional view of a coupling device according toa second embodiment of the present invention;

[0029]FIG. 4 is a cross sectional view showing the coupling device whena breakable portion (thinner portion) breaks, according to the secondembodiment; and

[0030]FIG. 5 is a cross sectional view showing a coupling deviceaccording to a third embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0031] Referring to FIG. 1, the present invention is applied to acoupling device 10 between an engine for propelling a vehicle and arotary machine mounted on the vehicle. The rotary machine is acompressor 20 for a refrigerant cycle. The refrigerant cycle is acomponent of a vehicular climate control system. The coupling device 10is attached on a shaft 22 of the compressor 22. The coupling device 10is provided with a pulley 11 and an electric motor 30.

[0032] The pulley 11 is made of metal and is formed in a doublecylindrical structure. The pulley 11 has an outer surface 11 a with apoly-V groove and is rotatably supported on a front housing 21 of thecompressor 20 via a radial bearing 12. The pulley 11 is coupled with acrankshaft of the engine via a poly-V belt. The pulley 11 is rotatedduring the engine is running.

[0033] A hub 13 is fixed on the pulley 11. The hub 13 has an outermember 14, an inner member 15 and a damper 16. The outer member 14 isformed in an annular shape and has an L-shaped cross section. The outermember 14 is bolted on the pulley 11. The outer member 14 is made of ametal plate such as a cold rolled steel plate, and is manufactured by apressing process. The inner member 15 is fixed on the shaft by a fixingmeans for fixing the inner member 15 on the shaft by the thrust forceapplied by threads. The damper 16 joins the outer member 14 and theinner member 15 for transmitting the torque. The damper 16 is made ofelastic material such as a rubber, e.g., a chlorinatedisobutylene-isoprene (Cl-IIR) rubber, and is joined with the inner andouter member 14, 15 by vulcanization.

[0034] The damper 16 works as a shock absorber for absorbing torquefluctuation. The damper 16 also works as an urging member for urging theinner member 15 in an axial direction to separate an inner ring 17 afrom an inner ring 18 a.

[0035] A first one-way clutch 17 is press fitted into a radial center ofthe inner member 15. The hub 13 and the first one-way clutch 17 providea transmission path from the pulley 11 to the shaft 22. An outer ring ofthe first one-way clutch 17 is fixed with the inner member 15. An innerring 17 a of the first one-way clutch 17 is coupled with the shaft 22 bytwo means.

[0036] The first is an axial frictional coupling. The inner ring 17 a isfrictionally coupled with the shaft 22 via a contact with the inner ring18 a.

[0037] The second is a thread coupling. The inner ring 17 a is coupledwith the shaft via a torque limiter 19 by screwing the torque limiter 19onto the shaft. The inner ring 17 a is fixed on torque limiter 19 via athread coupling and an axial contact on an outer cylindrical portion 19a of a torque limiter 19. The torque limiter 19 has an axial annularsurface 19 d on an axial end of the outer cylindrical portion 19 a sothat the surface 19 d comes in contact with an inwardly extending flangeportion 17 b of the inner ring 17 a. The threads on the inner ring 17 aand the outer cylindrical portion 19 a are formed in a direction totighten themselves when the engine drives the pulley 11. In other words,the threads are formed in a direction to tighten themselves in responseto a torque transmitted through the first one-way clutch 17 to thetorque limiter 19. Therefore, the inner member 15 and the torque limiter19 are rotated together during the engine is running.

[0038] The torque limiter 19 further has an inner cylindrical portion 19b. The inner cylindrical portion 19 b is threaded on the shaft 22. Theinner cylindrical portion 19 b has a female thread 19 c on its innersurface. The shaft 22 has a male thread 22 a on its outer surface. Thethreads 19 c, 22 a are formed in a direction to tighten themselves whenthe torque limiter 19 is tightened in a direction that is the same as atorque direction transmitted from the hub 13 to the shaft 22. Therefore,the threads 19 c, 22 a generate a thrust force when the threads 19 c, 22a are tightened. The threads 19 c, 22 a are primarily tightened with atorque that is greater than the required transmissible torque T3 and isless than the break torque T1.

[0039] The torque limiter 19 has a breakable portion 19 e between twoportions 19 a, 19 b. The first portion 19 a is coupled with the pulley11 via the inner ring 17 a and the hub 13. The other portion 19 b iscoupled with the shaft 22 via the threads. The breakable portion 19 e isalso located between the surface 19 d and the inner cylindrical portion19 b. The breakable portion 19 e is formed thinner than the otherportion of the torque limiter 19. The breakable portion 19 e has lesstensile strength against the thrust force, the force in an axialdirection, than that on the other portions of the torque limiter 19. Thebreakable portion 19 e has a tensile strength that is breakable at apredetermined level of the thrust force. The torque limiter 19 isintegrally formed with sintered metallic composite and has a coating ofmolybdenum disulfide at least on the female thread 19 c.

[0040] In this embodiment, it is desirable to keep frictionalcoefficients constant on the male thread 22 a, the female thread 19 cand contacting surfaces of the inner ring 19 d, since the break torqueof the breakable portion 19 e and the transmissible torque of thecoupling device may be affected with the frictional coefficients onthese portions. In order keep expected functions, the male thread 22 a,the female thread 19 c, the surface 19 d and a contact surfaces betweenthe inner ring 17 a and the outer ting 18 s are coated with themolybdenum disulfide coating. Alternatively, another rust preventivecoating may be used instead of the molybdenum disulfide coating.

[0041] The electric motor 30 is integrated in the coupling device 10 asan internal driving power source for driving the shaft 22. The electricmotor 30 is configured as an outer rotor type DC brushless motor. Theelectric motor 30 has a stator 31 with coils and a rotor 32 withmagnets. The stator 31 is fixed on the front housing 21 of thecompressor 20. The rotor 32 is disposed outside the stator 31 and isrotatably supported on the shaft 22. The rotor 32 is supported on theshaft 22 via a second one-way clutch 18. The second one-way clutch 18selectively transmits a torque in the same direction as that transmittedby the first one-way clutch.

[0042] The second one-way clutch 18 is fixed on an outer surface of theshaft 22 between the first one-way clutch 17 and a stepped portion 22 bof the shaft 22. The second one-way clutch 18 has an inner ring 18 a.The one end of the inner ring 18 a comes in contact with the steppedportion 22 b, and the other end of the inner ring 18 a comes in contactwith axially facing end surface of the inner ring 17 a. Therefore theinner ring 18 a provides an extension member of the shaft 22 to receivethe inner ring 17 a, which is directly tightened axially by threadingthe torque limiter 19.

[0043] In an assembling step, the torque limiter 19 is screwed into theinner ring 17 a and onto the shaft 22 like a nut. The torque limiter 19is further screwed onto the shaft 22 after the surface 19 d comes incontacts with the flange portion 17 b. The damper 16 enables the innerring 17 a to move axially. As the torque limiter 19 is screwed, thetorque limiter 19 applies the thrust force on the inner ring 17 a andthe inner ring 18 a. Then, the inner ring 17 a and the inner ring 18 aare come in contact with the stepped portion 22 b and pressed onto thestepped portion 22 b. The torque limiter 19 is tightened with a torquethat is greater than the required transmissible torque T3 and is lessthan the break torque T1. As a result, the inner ring 17 a, the innerring 18 a and the torque limiter 19 are fastened and fixed on the shaft22 by the thrust force, an axial tightening force. However, a contactbetween the inner ring 17 a and the inner ring 18 a, and a contactbetween the inner ring 18 a and the shaft 22 are merely frictional,therefore these components are still rotatable each other in a slidingmanner when a predetermined level of rotational torque is applied.

[0044] In an operation of the coupling device 10, the coupling device 10transmits a torque from the pulley 11 to the shaft 22, and in additionto or alternatively the electric motor 30 drives the shaft 22. Thepulley 11 receives rotational drive torque from the engine. The torqueis transmitted to the shaft 22 through the damper 16, the inner member15, the first one-way clutch 17 and the inner ring 18 a.

[0045] When the transmitted torque is lower than the tightening torqueof the torque limiter 19, the inner ring 17 a rotate with the shaft 22with no slip. Therefore, the breakable portion 19 e only receives astable stress caused by the tightening force of the torque limiter 19and the thrust force generated in accordance with the tightening force.The breakable portion 19 e receives very little or no stress caused bythe fluctuation of the transmitted torque.

[0046] Next, if the transmitted torque exceeds the tightening torque,the inner ring 17 a rotates with respect to the shaft 22 while slidingon the inner ring 18 a or sliding together with the inner ring 18 a onthe stepped portion 22 b. This rotation of the inner ring 17 a screwsthe torque limiter 19 onto the shaft 22, and advances the innercylindrical portion 19 b in the axial direction. At this time, the innercylindrical portion 19 b is advanced in the axial direction, however,the outer cylindrical portion 19 a is kept on the same axial position.Therefore, the tensile stress on the breakable portion 19 e is increasedas the inner ring 17 a rotates with respect to the shaft 22. In order toadjust the friction between the inner ring 17 a and the inner ring 18 a,the contacting portion therebetween is narrowed.

[0047] Then, if the inner ring 17 a rotates in a certain angle withrespect to the shaft 22, the tensile stress caused by screwing andtightening the torque limiter 19 breaks the breakable portion 19 e. Thebreakage of the breakable portion 19 e separates the torque limiter 19into two portions, the outer cylindrical portion 19 a and the innercylindrical portion 19 b. The inner ring 17 a is separated from theinner ring 18 a due to the axial urging force of the damper 16. As aresult, the torque transmission from the pulley 11 to the shaft 22 isdisconnected.

[0048] The second one-way clutch 18 prevents torque transmission fromthe shaft 22 to the rotor 32 during the shaft 22 is driven by the pulley11. The first one-way clutch 17 prevents torque transmission from theshaft 22 to the pulley 11 during the electric motor 30 drives the shaft22.

[0049] In this embodiment, the pulley 11, the hub 13 and the firstone-way clutch 18 provide a drive side member. The shaft 22 and theinner ring 18 a are a driven side member. The engine is a primary drivesource. The electric motor 30 is a secondary drive source.

[0050] According to the embodiment, since the transmitted torque is notdirectly applied on the breakable portion 19 e, the breakable portion 19e receives very little or no stress caused by the fluctuation of thetransmitted torque. The stress on the breakable portion 19 e is notfluctuated in accordance with the transmitted torque. Therefore, thebreakable portion 19 e is prevented from the metal fatigue failurecaused by repeated cycling of the load.

[0051] In another aspect, the breakable portion 19 e is designed tobreak in response to the tensile stress caused by the thrust forcerather than the shearing stress. Since the tensile stress shows asubstantially uniform distribution over a cross section, it is possibleto estimate the stress distribution on the breakable portion 19 eprecisely and to estimate the break torque precisely. As a result, it ispossible to reduce or eliminate the trial and error type development,and to reduce cost.

[0052] In another aspect, since the torque limiter 19 is provided as anindependent component, it is possible to adjust the break torqueprecisely. Further, since the breakable portion 19 e is formed in acylindrical shape having a diameter substantially the same as the shaft22, it is possible to make the torque limiter 19 in small and lessmaterial.

[0053] In another aspect, since the torque limiter 19 is axiallyseparated into two portions, it is possible to reduce or prevent noiseor undesirable friction after the breakage of the breakable portion 19e. In addition, a separating means for separating the portions 19 a, 19b is provided by the damper 16, therefore it is possible to reduce thenumber of parts.

[0054] In another aspect, the first and second one-way clutch 17, 18prevents the torque limiter 19 from loosening.

[0055] The present invention can be applied to the other couplingdevice, for example, to a coupling device that has no electric motor.Any type of one-way clutch may be used as the first and second one-wayclutch 17, 18, for example, a roller type and a sprag type can be used.The present invention can be applied to a coupling device that has apulley portion and an electromagnetic clutch for controlling the torquetransmission.

[0056]FIG. 3 shows a second embodiment of a coupling device according tothe present invention. The coupling device has a simplified structurecompared with the first embodiment. The coupling device 10 has somecomponents explained in the first embodiment, therefore the samereference numbers are used to indicate the same or similar components asthe first embodiment.

[0057] The shaft 22 is rotatably supported on the housing 20. The pulley11 is rotatably supported on the housing 20 via the bearing 12. Thepulley 11 is coupled with the hub 13 including the outer member 14, theinner member 15 and the damper 16. For example, the hub 13 and thepulley 11 are tightly fastened by a screw member 18. The inner member 15has a disk portion 15 d, an outer cylindrical portion 15 c and an innercylindrical portion 15 b. The disk portion 15 d has a seat portion 15 gresting on an end 22 b of the shaft 22. The inner cylindrical portion 15b has a thicker portion 15 e and a thinner portion 15 f as the breakableportion. The thinner portion 15 f is located between the seat potion 15g and the thicker portion 15 e. A thread 15 a (i.e., screw portion) isformed on the inner cylindrical portion 15 b. A thread 22 a (i.e., screwportion) is formed on the shaft 22.

[0058] In the second embodiment, a film made of molybdenum disulfide isformed on at least one of the seat portion 15 g and the end 22 b of theshaft 22, so that the coefficient of friction between the seat portion15 g and the end 22 b of the shaft 22 is made stable.

[0059] The threads 15 a, 22 a are formed in a right hand or left handfashion so as to thrust the seat surface 15 g onto the end 22 b of theshaft 22 when the inner cylindrical portion 15 b is rotated andtightened in a direction that is the same as a torque directiontransmitted from the hub 13 to the shaft 22. Therefore, the thrustpressure acting on the seat surface 15 g increases as the threads 15 a,22 a are tightened. The threads 15 a, 22 a are primarily tightened witha torque that is greater than the required transmissible torque and isless than the break torque.

[0060] The second embodiment may be used as a pulley for a refrigerantcompressor of a vehicular refrigerant cycle. However, since the secondembodiment has no one-way clutch, there is the possibility that thethreads 15 a, 22 a may be loosened if a torque opposite to the torquedirection from the hub 13 to the shaft 22 is applied to the threads 15a, 22 a.

[0061] According to the second embodiment, the thinner portion 15 freceives the shearing stress caused by a tightening torque applied whentightening the inner cylindrical portion 15 b and the tensile stresscaused by a thrust force generated by the threads 15 a, 22 a. When thetransmitted torque exceeds the primary tightening torque, the threads 15a, 22 a are tightened. The threads 15 a, 22 a are proposed the innercylindrical portion 15 b in accordance with the transmitted torque and atime period for which the transmitted torque exceeds the predeterminedtorque. Therefore, the tensile stress is gradually increased and thenbreaks the thinner portion 15 f.

[0062] The thinner portion 15 f receives very little amount of thefatigue stress caused by a fluctuation of the transmitted torque.Therefore, the thinner portion 15 f is avoided from the metal fatiguefailure during transmitting torque normally since a variation of thetransmitted torque does not affect the stress on the thinner portion 15f. As a result, it is possible to break the thinner portion 15 f onlywhen an excessive torque is applied. FIG. 4 shows a state where thethinner portion 15 f is broken when a thrust force larger thanpredetermined torque is applied to the thinner portion 15 f.

[0063] According to the second embodiment of the present invention, theseat portion 15 g is used as a contact surface that contacts the end 22b of the shaft 22, and the thinner portion 15 f is disposed between apart of the thread 15 a and the seat portion 15 g. In the secondembodiment, the thread 15 e is also provided on the thinner portion 15f. However, the thread 15 e provided on the thinner portion 15 f can beomitted. In this case, the thinner portion 15 f can be positionedbetween the thread 15 a and the seat portion 15 g.

[0064] In the second embodiment, the damper 16 can be used as aseparation member that is disposed to separate the disk portion 15 d andthe like from the shaft 22 when the thinner portion 15 f breaks. Becausethe damper 16 is an elastic member for applying elastic force to atleast one of the disk portion 15 d and the draft 22, it can preventnoise in the broken portion of the thinner portion 15 f from beingcaused, and prevent an unnecessary friction resistance from beingcaused.

[0065] A third embodiment of the present invention will be now describedwith reference to FIG. 5. In the above-described second embodiment, afilm made of molybdenum disulfide is formed on the seat portion 15 g andthe end 22 b of the shaft 22, so that the coefficient of frictionbetween the seat portion 15 g and the end 22 b of the shaft 22 is madestable. However, in the third embodiment, as shown in FIG. 5, a spacer17, on which a film of molybdenum disulfide is formed, is disposedbetween the disk portion 15 d and the end 22 b of the shaft 22.Therefore, the spacer 17 restricts a variation in the coefficient offriction.

[0066] Although the present invention has been described in connectionwith the preferred embodiments thereof with reference to theaccompanying drawings, it is to be noted that various changes andmodifications will be apparent to those skilled in the art.

[0067] For example, in the above-described second and third embodiments,a female screw can be formed as the thread 15 a on the inner peripheralsurface of the cylindrical portion 15 b (the hub 13), and a male screwcan be formed as the thread 22 a on a driven rotational member (i.e.,shaft 22). Conversely, the male screw can be formed as the thread 15 aon the cylindrical portion 15 b (i.e., the hub 13), and the female screwcan be formed as the thread 22 a on the driven rotational member (i.e.,shaft 22).

[0068] In the above-described second and third embodiments, the hub 13is directly connected to the shaft 22. However, a driven rotationalmember rotated integrally with the shaft 22 may be provided. In thiscase, the hub 13 of the drive rotational member is connected to thedriven rotational member that is rotated integrally with the shaft 22.

[0069] In the above-described second and third embodiments, thecylindrical portion 15 b including the breakable portion (i.e., thethinner portion 15 f) is made of a sintered metal. However, thecylindrical portion 15 b including the thinner portion 15 f can be madeof the other material such as ceramic. Further, the thread 15 a (screwportion) is provided substantially on an entire inner surface of thecylindrical portion 15 b including the thinner portion 15 f. However,the thread 15 a can be omitted on the inner surface of the thinnerportion 15 f, among the cylindrical portion 15 b.

[0070] Such changes and modifications are to be understood as beingincluded within the scope of the present invention as defined in theappended claims.

What is claimed is:
 1. A coupling device disposed between a drive member and a driven member, the coupling device comprising: a torque limiter disposed to be rotated with the drive member, the torque limiter having a thread formed in a direction to generate a thrust force when the thread is rotated in the same direction as a direction of a torque transmitted from the drive member to the driven member, and a breakable portion having a tensile strength that breaks at a predetermined level of the thrust force.
 2. The coupling device according to claim 1, further comprising a one-way clutch that prevents the thread from loosening by a torque in a counter direction.
 3. The coupling device according to claim 1, wherein the breakable portion has the tensile strength that is less than the other portions of the torque limiter.
 4. The coupling device according to claim 1, wherein the torque limiter is fixed on the drive member, and the thread is adapted to be threaded onto the driven member to fasten the drive member on the driven member in a slidable manner.
 5. The coupling device according to claim 4, wherein the thread is formed to make the drive member comes in contact with the driven member in a manner that the drive member directly transmits torque to the driven member when the transmitted torque is less than a predetermined level, and that the drive member is enabled to rotate with respect to the driven member when the transmitted torque exceeds the predetermined level.
 6. The coupling device according to claim 4, wherein the torque limiter is formed in a cylindrical shape that has a first portion to be fixed on the drive member, and a second portion to be threaded on the driven member, wherein the breakable portion is disposed between the first and second portions.
 7. The coupling device according to claim 4, wherein the torque limiter is threaded on the drive member until the torque limiter and the drive member comes in an axial contact.
 8. The coupling device according to claim 1, further comprising a member that urges the torque limiter opposite to the tightening direction of the thread.
 9. The coupling device according to claim 1, wherein the drive member includes: a first one-way clutch that selectively transmits a torque from a primary drive source to the driven member; and a second one-way clutch that selectively transmits a torque from a secondary drive source to the driven member, wherein the torque limiter is disposed to be rotated by the torque from the primary or secondary drive source.
 10. A coupling device disposed between a drive member and a driven member, the coupling device comprising: a torque limiter disposed to be rotated with the drive member, the torque limiter having a thread formed in a direction to generate a thrust force when the thread is tightened in the same direction as the torque direction transmitted from the drive member to the driven member, and a breakable portion having a tensile strength that breaks at a predetermined level of the thrust force; and a one-way clutch that prevents the thread from loosening by a torque in counter direction.
 11. A coupling device comprising: a first one-way clutch that selectively transmits torque from a first power source to a driven member; a second one-way clutch that selectively transmits torque from a second power source to a driven member; and a torque limiter having a thread formed in a direction to generate a thrust force for fixing a member of the first one-way clutch and a member of the second one-way clutch on the driven member when the thread is tightened in the same direction as the torque direction transmitted from the first and second power source to the driven member, and a breakable portion having a tensile strength that breaks at a predetermined level of the thrust force, wherein the torque limiter is disposed to be rotated by a torque from one of the first and second power sources.
 12. A coupling device disposed between a drive member and a driven member, the coupling device comprising: a torque limiter disposed to be rotated with the drive member, the torque limiter having: a contact surface disposed to contact the driven member; a thread that is provided to generate a trust force in a contacting direction for press-contacting the driven member and the contact surface, when the thread is rotated in the same direction as a direction of torque transmitted from the drive member to the driven member; and a breakable portion disposed between a part of the thread and the contact surface, wherein the breakable portion breaks when the torque is larger than a predetermined value.
 13. The coupling device according to claim 12, further comprising a separation member that is disposed to separate the torque limiter from the driven member when the breakable portion breaks.
 14. The coupling device according to claim 12, further comprising an elastic member that is disposed to apply an elastic force to at least one of the torque limiter and the driven member, in a direction opposite to the contacting direction
 15. The coupling device according to claim 12, wherein: the contact surface of the torque limiter contacts one end of the driven member; at least one of the contact surface and the one end of the driven member has a film on its surface; and the film restricts a variation in coefficient of friction when the one end of the driven member contacts the contact surface.
 16. The coupling device according to claim 12, further comprising: a spacer having a surface that restrict a variation in coefficient of friction, the spacer being disposed between the contact surface of the torque limiter and the driven member. 